This application claims the priority of German Patent Application, Serial No. 199 48 831.2, filed Oct. 6, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a mobile crane, and more particularly to a truck crane of a type having a driven undercarriage and a superstructure rotatably mounted on the undercarriage for supporting a counterweight, a slewable boom assembly and an operator""s cab.
Small mobile cranes with a load-carrying capability of less than 100 tons are frequently used in closed halls for lifting machine components. Therefore, mobile cranes should exhibit a compact design, i.e. small dimensions, while still exhibiting maneuverability, especially when requiring travel in tight radii inside the hall. Moreover, a low emission of pollutants is demanded, in particular when the crane is used in narrow city areas such as pedestrian zones.
An example of a conventional mobile crane is shown in FIG. 1. The mobile crane includes an undercarriage or chassis 1 which is supported for mobility on wheels 35 fitted to front and rear axles 2, 3. Mounted on the undercarriage 1 for rotation is a superstructure 4 which supports a telescopic boom assembly made up of a base section 5 and three boom sections 6, 7, 8 fitting into each other. The smallest boom section 8, called fly section, carries the boom head 9 for guiding the hoisting rope (not shown). The superstructure 4 further carries a counterweight 10 and an operator""s cab 11. The drive mechanism for propelling the wheels 35 in an operating direction includes a conventional diesel engine 12 which has attached thereto a transmission 13 connected via a cardan shaft 14 to a transfer case 15 for distributing the driving power of the diesel engine 12 via two cardan shafts 16, 17 to bevel gearings 18, 19 arranged on the axles 2, 3.
Conventional mobile cranes of this type thus have a drive train comprised of diesel engine, automatic transmission, transfer case, cardan shafts. In addition, hydraulic pumps are provided to implement crane operations. These cranes suffer, however, shortcomings because all these components can be installed only in specific locations so that the degree of freedom of placement is very restricted. Shifting operations inevitably result in engine-torque jumps and transient operating states that are determinative for the toxic constituents contained in the exhaust gas.
It is thus an object of the present invention to provide an improved mobile crane, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved mobile crane which provides a high degree of freedom for installation of components of the drive train and operates at substantially zero or at least very low emission of pollutants.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing an undercarriage, a superstructure rotatably mounted on the undercarriage for supporting a counterweight, a slewable boom assembly and an operator""s cab, at least one electric motor for propelling the undercarriage in an operating direction, and a power supply unit for supply of electric power for driving and crane operations, whereby the power supply unit is drivingly connected via a cabling to the at least one electric motor and can be configured as a diesel-electric drive, or a battery, or a multi-system drive formed by a combination of a diesel-electric drive with a battery, or a fuel-cell system.
In accordance with the present invention, the propulsion of the crane is provided by at least one electric motor whereas the power supply for the driving and crane operations is assumed by a diesel-electric drive, or a battery, or a multi-system drive formed by a combination of a diesel-electric drive with a battery, or a fuel-cell system, connected to the at least one electric motor via a cabling. A supply of electric power through use of a high-performance battery alone results in a zero emission of pollutants because the battery can be re-charged via a mains power outlet. Thus, by utilizing the battery only, crane works as well as driving operation can be done without emission of pollutants.
When electric power is supplied by the multi-system drive, it is suitable to carry out the driving and crane operations predominantly by the diesel-electric drive and only temporarily by the battery. The use of a fuel-cell system as power supply unit results in water as waste product so that this variation essentially results also in zero emission of pollutants.
According to another feature of the present invention, the undercarriage is supported on wheels for movement in the operating direction, whereby each wheel is driven by an electrically operated wheel-hub motor. Another option is to connect to each of the front and rear axles, to which the wheels are fitted, with a separate electric motor. In some cases, it may be suitable to use only a single electric motor which is drivingly connected to the axles via a transfer case and cardan shafts.
As a result of a separate power supply unit and the provision of an electric motor for driving and crane operations the crane, the wheels can be mechanically de-coupled from the diesel engine, thereby realizing a greater freedom of installation of the drive train. For example, the diesel-electric drive may be located in a rear portion of the undercarriage, and thereby used as part of the counterweight. The same is true for the battery or the fuel-cell system which can be disposed as a separate counterweight on the superstructure. Another option may be the placement of the diesel-electric drive and the battery in the superstructure as part of the counterweight.
The provision of an electric motor for driving and operating the crane results in a smooth crane operation and permits an optimum dimensioning of the diesel engine as the diesel engine can run continuously in the most favorable speed range. As a consequence, the emission of pollutants decreases accordingly.
According to another feature of the present invention, the diesel-electric drive may be used only for drive-in of the mobile crane into a hall and to subsequently connect the crane in the hall to a power outlet of the electric mains, provided the electric infrastructure exists, so that the crane operation can be carried out without emission of pollutants.